In flight

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In baseball, the rules state that a batted ball is considered in flight when it has not yet touched any object other than a fielder or his equipment. Such a ball can be caught by a fielder to put the batter out.

Once a batted ball touches the ground, a fence or wall, a foul pole, a base, the pitcher's rubber, an umpire, or a baserunner, it is no longer in flight. A batted ball that passes entirely out of the playing field ceases to be in flight when that occurs; if it was between the foul poles at that moment, then it is a home run which entitles the batter (and any other runners on base) to score.

A special rule exists in covered baseball facilities (retractable or fixed roofed), where a batted ball striking the roof, roof supporting structure, or objects suspended from the roof (e.g., speakers) while in fair territory is still considered to be in flight. Rules for batted balls striking any of those objects in foul territory differ between ballparks, with most considering such a ball to still be in flight, and some considering it to be a foul ball and dead from the time it strikes.